1. Field of the Invention
This invention pertains generally to audio amplifiers, and more particularly to a solid state audio amplifier having a low impedance output which can directly drive a speaker without use of an impedance matching transformer.
2. Description of the Background Art
Audio amplifiers are widely used in various applications. Such amplifiers can be constructed using vacuum tubes or solid state devices.
Despite the availability of solid state amplifiers, vacuum tube amplifiers are still popular because of their substantially linear amplification characteristics. A very basic vacuum tube audio amplifier can be seen in FIG. 1. The triode amplifier of this circuit produces a substantially linear output. To the extent that there is non-linearity in the output, the amplifier will produce even order harmonic distortion which is more acceptable to the ear than odd order harmonic distortion. When driven into saturation, instead of sharp clipping, the amplifier rolls off. A drawback of vacuum tube amplifiers, however, is that they are low current, high voltage devices. Therefore, a bulky, expensive and lossy output transformer is required to drive a speaker.
A solid state equivalent of the vacuum tube audio amplifier of FIG. 1 can be seen in FIG. 2, where a bipolar junction transistor has been substituted for the vacuum tube. Transistors, unlike vacuum tubes, however, are high current devices that can produce sufficient power to drive speakers without a transformer. FIG. 3 shows a typical circuit of a transformerless equivalent of the bipolar transistor audio amplifier shown in FIG. 2, where I.sub.S represents a constant current source. This amplifier configuration exhibits soft saturation and moderately good linearity. However, the non-linearities are due to the exponential characteristics of the bipolar junction transistor and result in mostly odd order harmonic distortion. Further, the output impedance is effectively the output impedance of the bipolar junction transistor, which is typically on the order of several thousand ohms. Because the impedance of this amplifier does not match that of a typical speaker, which is on the order of a few ohms, the circuit shown in FIG. 3 is not widely used. FIG. 4, however, shows an example of the most commonly used transformerless bipolar junction transistor audio amplifier configuration. While this configuration exhibits linear amplification and relatively low output impedance due to the feedback between the base and emitter, when the circuit is over-driven the saturation is very sharp and rich in odd order harmonic distortion. By using a typical field effect transistor (FET) instead of a bipolar transistor in the circuit of FIG. 3, odd order harmonic distortion can be minimized because a field effect transistor is a square-law device which will exhibit mostly second harmonic distortion when operating in its linear mode. However, the output impedance of such amplifiers is on the order of several thousand ohms which is very high compared to the speaker impedance. Therefore, an impedance mismatch still exists. An FET can likewise be substituted for the bipolar transistor in the circuit of FIG. 4; however, the saturation behavior still remains sharp and rich in odd order harmonic distortion.
As can be seen from the foregoing, while transistor amplifier circuits can be designed with some of the desirable characteristics of vacuum tube amplifiers with regard to linearity, saturation characteristics, and output impedance, there is presently no transistor amplifier which exhibits triode vacuum tube amplifier characteristics while, at the same time, has a low output impedance which can drive a speaker directly. Those transistor amplifiers which mostly closely exhibit triode vacuum tube amplifier linearity and saturation characteristics, typically have output impedances of several thousand ohms. However, speakers have a nominal impedance of eight ohms which, in actuality, typically ranges from approximately two to sixteen ohms. Since maximum power transfer occurs when the impedance of the amplifier matches the impedance of the load, it is necessary to use an impedance matching transformer between the output of such amplifiers and the speaker. Use of such transformers, however, adds weight and cost to the amplifier. In addition, transformers are lossy circuit elements and waste power.
Therefore, there is a need for a solid state amplifier which emulates the linearity and saturation characteristics of a vacuum tube amplifier, while at the same time has an output impedance which nominally matches the impedance of a speaker. The present invention satisfies that need, as well as others, and overcomes the deficiencies in previously known amplifiers.
The foregoing description of background art is tendered for the purpose of disclosing information which may be pertinent in the examination of this application. It is respectfully stipulated, however, that none of the forgoing information, when considered singly or in combination, anticipates or renders the applicant' inventions obvious.